The present invention relates to burner assemblies, and particularly to oxygen-fuel burner assemblies. More particularly, the present invention relates to a burner having a fuel-delivery system and a staged oxygen-supply system.
One challenge facing the burner industry is to design an improved burner that produces lower nitrogen oxide emissions during operation than conventional burners. Typically, an industrial burner discharges a mixture of fuel and either air or oxygen. A proper ratio of fuel and air is established to produce a combustible fuel and air mixture. Once ignited, this combustible mixture burns to produce a flame that can be used to heat various products in a wide variety of industrial applications. Combustion of fuels such as natural gas, oil, liquid propane gas, low BTU gases, and pulverized coals often produce several unwanted pollutant emissions such as nitrogen oxides (NO.sub.x), carbon monoxide (CO), and total hydrocarbons (THC).
Burners that combine oxygen with an atomized fuel and oxygen mixture to produce a combustible mixture are known. See, for example, U.S. Pat. No. 5,092,760 to Brown and Coppin. Burners having oxygen-enrichment systems are also known as disclosed in the IHEA Combustion Technology Manual, Fourth Edition (1988/), pp. 320-21, published by The Industrial Heating Equipment Association of Arlington, Va.
Burners were developed to burn a mixture of fuel and pure oxygen in an attempt to lower the amount of NO.sub.x produced during combustion. Regular combustion air contains a lot of nitrogen (N.sub.2) and pure oxygen contains no nitrogen. It has been observed that the higher flame temperatures brought on by burning a mixture of fuel and pure oxygen has caused the conversion of fuel-bound N.sub.2 into NO.sub.x to increase.
A burner assembly designed to burn fuel more completely using a lower flame temperature would lead to lower nitrogen oxide emissions. What is needed is a burner assembly that is able to burn a fuel and oxygen mixture without generating a lot of unwanted nitrogen oxide emissions.
According to the present invention, a burner assembly is provided for combining oxygen and fuel to produce a flame. The burner assembly includes a burner block formed to include a combustion chamber having inlet and outlet openings and a nozzle positioned to discharge an oxygen and fuel mixture into the combustion chamber through the inlet opening.
Means is also provided for supplying supplemental oxygen to the combustion chamber through the inlet opening. This supplemental oxygen mixes with the oxygen and fuel mixture in a first stage region inside the combustion chamber. This combustible mixture can be ignited to define a flame having a root portion in the combustion chamber and a tip portion outside the combustion chamber.
The burner block is also formed to include oxygen-discharging means around the outlet opening of the combustion chamber and oxygen-conducting means for conducting oxygen along one or more paths through the burner block and outside of the combustion chamber to the oxygen-discharging means. Means is also provided for delivering oxygen into the oxygen-conducting means formed in the burner block so that it passes through the oxygen-discharging means and is ejected from the burner block into a downstream second stage region containing a portion of the flame and lying outside the combustion chamber.
In preferred embodiments, the burner block is made of a refractory material and includes an outside wall formed to include the combustion chamber inlet opening and a plurality of oxygen-admission ports around the inlet opening. The burner block also includes a furnace wall configured to lie in a furnace and formed to include the combustion chamber outlet opening and a plurality of oxygen-discharge ports around the outlet opening. Illustratively, the burner block is also formed to include a plurality of oxygen-conducting passageways. Each passageway extends through the burner body to connect one of the oxygen-admission ports to one of the oxygen-discharge ports. Essentially, these passageways are arranged to bypass the combustion chamber and deliver oxygen to the second stage region downstream of the combustion chamber. Illustratively, the second stage region lies in a furnace adjacent to the burner block and the flame produced by the burner assembly heats products in the furnace.
An oxygen-supply manifold is provided to hold temporarily a supply of pressurized combustion oxygen for use in the burner assembly. In use, a continuous stream of pressurized oxygen is admitted into the oxygen-supply manifold using any suitable means. Some of that pressurized oxygen is distributed to the first stage region by the supplying means and the rest of that pressurized oxygen is distributed by the delivering means to the second stage region using the oxygen-conducting passageways formed in the burner block.
The supplying means is defined by an annular channel that extends along the longitudinally extending central axis of the nozzle and between the oxygen-supply manifold and the combustion chamber. This annular channel provides a flow path for distributing some of the pressurized oxygen in the oxygen-supply manifold to the first stage region to mix with the atomized oxygen and fuel mixture discharged by the nozzle.
Illustratively, the delivering means includes a plurality of oxygen-delivery tubes arranged to connect the oxygen-supply manifold to the oxygen-conducting passageways formed in the burner block. Each oxygen-delivery tube includes inlet means for receiving oxygen extant in the oxygen-supply manifold and outlet means for discharging oxygen into the oxygen-conducting passageways through the oxygen-admission ports. This oxygen then passes through the passageways and is ejected into the second stage region through the oxygen-discharge ports. It is within the scope of this invention to employ one annular oxygen-discharge port rather than a plurality of spaced-apart oxygen-discharge ports.
The burner assembly in accordance with the present invention introduces combustion oxygen into two regions or combustion zones. The first stage combustion zone is near the root of the flame inside the combustion chamber and the second stage combustion zone is in the furnace itself in a location downstream from the combustion chamber and nearer to the tip of the flame. Advantageously, by withholding a portion of the combustion oxygen from the root of the flame, the fuel partially burns and the fuel-bound nitrogen is converted into reducing agents. These nitrogenous compounds are subsequently oxidized to elemental nitrogen, thereby minimizing the generation of fuel nitrogen oxides. Also, the peak flame temperature is lowered in the fuel-rich first stage combustion zone since the generated heat dissipates rapidly. This reduction in flame temperature reduces the formation of nitrogen oxides which are temperature-dependent. In the second stage combustion zone, additional oxygen is injected through the burner block oxygen-discharge ports to complete combustion and optimize flame shape and length.
Additional features and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived.